This invention relates to compression/expansion refrigeration, and is particularly concerned with turbine-expansion cycle chiller, air conditioning, heat pump, or refrigeration systems in which a turbo-expander is employed to expand the condensed refrigerant to a reduced pressure and to permit recovery of a portion of the energy of the compressed fluid.
Single-fluid two-phase flow systems typically incorporate an expansion valve, float valve, or other mechanical pressure regulator between the condenser heat exchanger and the evaporator heat exchanger to expand the fluid, i.e., to throttle the flow of refrigerant fluid from a high pressure to a low pressure.
The use of a turbine or turbo-expander in a refrigeration cycle has been previously proposed with the aim of improving the refrigeration efficiency. Some type of bi-phase flow turbine is required to replace the isenthalpic expansion process of a throttling expansion valve with an isentropic expansion process. This is, the turbine absorbs some of the energy of the expanding refrigerant and converts it to rotational energy. At the same time, the liquid fraction of the refrigerant that enters the evaporator is increased. Ideally, the energy of the expanding refrigerant can be recovered and can be used to reduce the amount of motor energy needed to drive the system compressor.
U.S. Pat. No. 4,336,693 describes a refrigeration system that employs a reaction turbine as an expander stage. In this approach, a centrifugal reaction turbine preforms the expansion function, and operates to separate vapor from the liquid before extracting power. This produces increased efficiency over a conventional turbo-expander. In this prior patent, the energy produced by the turbine can be used to drive a load, such as a generator.
However, turbines placed in this role have not been particularly efficient for a number of reasons. In most refrigeration processes, where refrigerant is brought from a saturated liquid phase to a low-quality two-phase liquid/vapor state, the expansion process produces a relatively small amount of work, compared to the work input required for the compressor. Moreover, turbines that have been conventionally employed are not only smaller in capacity than the compressor, but also operate under conditions of low efficiency due to the two-phase flow and speed of the expanding fluid. For optimal efficiency, the two-phase flow turbines also require a completely different speed from the compressor. Consequently, the conventional engineering practice is not to employ a turbine expander because the small amount of savings in energy recovery and efficiency gains are far outweighed by the reduced initial and maintenance costs of a throttling valve.
A single fluid, two-phase-flow turbine expander can be made practical and efficient only if critical relationships of the turbine to the rest of the refrigeration system are observed. Direct coupling of the turbine rotor shaft to the drive of the compressor is possible if the turbine rotor has a design speed that permits it to serve as a high-efficiency expander, the turbine matches the properties of the refrigerant, such as vapor density and two-phase flow acoustic velocity, and the capacity of the refrigeration system (i.e., refrigerator, chiller or air conditioner) satisfies optimal mass flow conditions of the turbine expander. However, no previous system has observed these criteria, and so the desired efficiency increases have not been achieved.
For medium- to high-pressure refrigerants such as R134A and R22, two-phase flow turbo-expanders can be employed, of the type described e.g., in Ritzi et at., U.S. Pat No. 4,298,311, Hays et at. U.S. Pat. No. 4,336,693 and Hays et at. U.S. Pat. No. 4,438,638. These patents relate to turbines driven by a two-phase working fluid where most of the fluid mass (e.g., 90%) is liquid, and one or more nozzles directs the condensed refrigerant at a rotor so that the vapor and liquid mixture impacts the rotor. These turbines are designed as reaction turbines, so that kinetic energy of the expanding vapor is transformed into kinetic shaft output energy rather than into heat. This, in theory, maximizes the liquid fraction of the total mass of the working fluid after expansion.
However, in any given application, the size of the turbine that provides optimal expansion will not provide suitable output shaft power. The turbine's expansion capacity for a given mass flow should be matched with the required shaft speed to permit direct coupling to the compressor drive.
Turbine-expansion cycle refrigeration systems assume a normal, steady-state flow rate and pressure head condition. Under normal conditions, the condenser drain mass flow passes through the two-phase flow turbine expander, and expansion energy is transferred to the compressor drive train. This produces a reduction in the shaft horsepower requirement for the compressor.
Where the turbo-expander is designed as a fixed-geometry device, the turbo expander can operate efficiently over a given range of mass flow and pressure head conditions. These turbo expanders are designed to operate with liquid flow at a given rate and pressure reaching the nozzles. Problems may occur if the refrigeration system is operated at off-design conditions.
At off-design conditions, the pressure head may be too small or the flow rate may be too large to pass the condensed refrigerant efficiently through the turbine nozzles, thus starving the cooler or evaporator stage. If the system head drops, but mass flow is at the design flow rate or higher, the pressure for that volume of flow may be too low to pass the required liquid flow through the turbine nozzles. Liquid refrigerant then stacks up in the sump region of the condenser, starving the evaporator. This condition can force the system to shut down due to low cooler or evaporator pressure.
Accordingly, it is necessary to incorporate some additional means to permit a stable level of refrigerant to be maintained, even when operating outside the design pressure and mass flow conditions, but without interfering with the operation of the turbo-expander.